1. Field of the Disclosure
The disclosure relates to methods of using systems, and more particularly to methods of using systems including assemblies exposed to cryogenic regions.
2. Description of the Related Art
FIG. 1 includes a schematic drawing of a conventional magnetic resonance imaging (“MRI”) system 100 that includes a superconducting magnet 190 that is contained within a vessel 140. The vessel 140 includes a shell having an outer wall 142 and an inner wall 144. The outer wall 142 is exposed to an ambient 162 that includes air substantially at room temperature (approximately 295 to 305 K) and atmospheric pressure (approximately zero gauge pressure). An interior space 160 lies within the inner wall 144. The vessel 120 can include another wall 172. The superconducting magnet 190 can be in its superconducting state by at least partial immersion of the superconducting magnet 190 within a bath of liquid cryogen (below line 170), typically helium. A thermal shield 182 is disposed between the outer and inner walls 142 and 144.
The MRI system 110 also includes a patient wall 174 with a space 176 in which a patient (not illustrated) may be placed when using the MRI system 110 during normal operation. The MRI system 100 also includes a cryocooler 120, which is described in more detail with respect to FIG. 2. The MRI system 100 further includes an exhaust port 132 and an exhaust assembly 130 connected to an exhaust.
The MRI system 100 still further includes a controller 110 that controls operation of the MRI system 100, including the superconducting magnet 190, the cryocooler 120, the exhaust assembly 130, potentially one or more other components (not illustrated), or any combination thereof of the MRI system 100. Although not illustrated, more than one controller 110, cryocooler 120, exhaust assembly 130, or any combination thereof may be used. The controller 110 can be bi-directionally coupled (illustrated by the double-headed arrows in FIG. 1) to the superconducting magnet 190, the cryocooler 120, the exhaust assembly 130, one or more of the potentially other components, or any combination thereof. In another embodiment, one or more of the bi-directional couplings may be replaced by uni-directional couplings. In addition, the controller 110 may be replaced by or used in conjunction with a different controller 110 when servicing the MRI system 100. The schematic drawing in FIG. 1 is merely to better illustrate the methods as described herein, and therefore, other features, such as electrical feedthroughs for supplying power to the superconducting magnet 190, are not illustrated.
FIG. 2 includes a schematic drawing of a portion of the MRI system 100 that includes the cryocooler 120. The cryocooler 120 includes a housing 222 and a cooling sub-assembly 224. The cooling sub-assembly 224 can include a piston, a linkage to an external portion 226, a motor 228, another suitable component, or any combination thereof. A flange 230 is attached to the housing 222 and a bellows seal 232. The bellows seal 232 may be attached to the outer wall 142. The ambient 162 lies outside the bellows seal 232 and adjacent to the outer wall 142. The flange 230 can be attached to the outer wall 142 using securing elements 234 that extend through holes 236 in the flange 230. The securing elements 234 can include bolts, screws, nuts, springs, or any combination thereof.
The MRI system 100 also includes a transitional wall 246 disposed between the outer wall 142 and the inner wall 144. The transitional wall 246 includes a relatively wider portion closer to the outer wall 142 and a relatively narrower portion closer to the inner wall 144. After the securing elements 234 are tightened, the housing 222 can be in contact with a surface of the transitional wall 246 at area 248. A flexible portion 250 within the transitional wall 246 may allow for movement between the outer wall 142 and the inner wall 144. The transitional wall 246 is thermally connected to the thermal shield 182 using a thermally conductive element 284. In one particular embodiment, a copper braided strap may be used for the thermally conductive element 284 and may be attached to both the thermal shield 182 and the transitional wall 246 near the area 248. Thermal elements 238 are used to heat the housing 222 during servicing and are attached to the housing 222. A manual valve 290 is used to allow vapor from the transitional space 264 to flow to the exhaust.
During normal operating conditions, the internal space 160 may be maintained at a temperature in a range of approximately 3 to 5 K, and a transitional space 264 may have a temperature that varies from approximately 40 K closer to the area 248 of the transitional wall 246 and another temperature closer to room temperature near the flange 230. The cryocooler 120 is exposed to the cryogen within the cryogenic region.
A conventional method of servicing a cryocooler for an MRI system can include removing the entire cryocooler. Before removing the cryocooler, the superconducting magnet is ramped down to a substantially zero magnetic field, and the internal space surrounding the superconducting magnet is de-pressurized. The housing of the cyrocooler may be heated to a temperature above the freezing point of water (approximately 273 K). Heating elements within the cyrocooler can be activated and controlled by an external controller to heat the housing and increase the temperature to reduce the likelihood of ice formation. When the cryocooler is thermally connected to thermal shield, the heating has to compensate for the heat sink effect of the thermal shield, and thus a substantially amount of energy may be consumed to heat the cryocooler and the thermal shield.
The liquid cryogen used to maintain the superconducting state of the superconducting magnet can be susceptible to heat and air. Heat can enter the interior space during servicing both from warm (ambient) air that enters, and by heat that is conducted along parts of the MRI system that remain (e.g., along the transitional wall, the inner wall, and potentially other thermally conductive elements within magnet subsystem). If the heating, whether by air or thermal conduction, is significant a quench event can occur, causing almost all of the cryogen to boil off nearly instantaneously. Thus, too much heating can be dangerous.
Additionally, room air can cause problems. If a component, such as the cryocooler, is exposed to air at room temperature and later taken below 60 K, ice can form. When liquid helium is used as the cryogen, ice can come from moisture, nitrogen, oxygen, argon, or any combination thereof. Ice can reduce the likelihood of forming a good thermal connection between the cryocooler and the transitional wall or other portions (e.g., a thermal shield) that are to be cooled, thus, substantially decreasing the efficiency of the cryocooler after servicing.
Still further, a significant amount of cryogen can be lost by ramping down the superconducting magnet to substantially zero field and ramping up the superconducting magnet back to its normal operating field. Additionally, ramping down and then ramping back up takes time and requires specialized equipment and trained personnel, thereby increasing the unavailability of the MRI system and the cost of the service.
The use of the same reference symbols in different drawings indicates similar or identical items. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.